Flattening Annotations
Flattening annotations is the act of painting the annotations directly on the page to which they belong so they are not editable anymore. This will permanently write an annotation into the document. For instance, a custom signature stamp can be flattened so the signature is added to the document itself and doesn't sit on a document as an annotation anymore.
Note: The following annotations (file attachment annotations, links, sounds, and redaction annotations) will NOT be flattened when flattening annotations. Even though technically file attachment annotations, links, sounds, and redaction annotations are annotations, they contain actions that would also be removed if the annotation was flattened and so they remain to not interfere with the functions placed in the PDF.
Flatten a Specific Annotation:
- Using the Hand Tool
or Object Selection Tool 
Select the annotation that you wish to flatten. - Right-click on the annotation (Ctrl + Click on Mac), choose "Flatten" in the context menu.
Flatten Multiple Annotations:
From the Comments Tab choose 
Flatten
Options
Pages - The range of pages to be exported as images
- All Pages: all pages in the document
- Current: only the current page in view
- Page Range: set the pages to be exported using the From and To options
- Even Pages Only: only export the even pages of the document
- Odd Pages Only: only export the odd pages of the document
Allow Unflattening - When checked allows the flattening process to be reversed at a later time. Flattening is irreversible if the PDF is flattened without this option checked.
NOTE: Any additional document or content changes performed on any page could affect unflattening so it is recommended as a final step before saving the document.
Assign to Layer - Adds the comment to the selected layer before flattening.
All Types - Flattens all annotations found in the document
Selected Types - Flattens only the annotation types checked in the list. Any annotations not checked will remain as an interactive annotation.